The intestinal mucosa is continuously exposed to a myriad of microorganisms and environmental antigens. Dysregulation of intestinal immune cells can potentially lead to inflammatory bowel disease (IBD) and Celiac disease. Hence, there is a critical need for balance between immunity against pathogens and tolerance to commensal organisms and innocuous food antigens. Co-stimulatory and co-inhibitory pathways control the balance between pathogenic and protective immune responses. The great interest in identifying therapeutic targets for IBD and other intestinal disorders gives impetus to further investigate how co-inhibitory molecules regulate intestinal homeostasis, inflammation and immunity. Our preliminary data lead us to hypothesize that CD160, a co-inhibitory molecules expressed on intraepithelial lymphocytes (IEL) and innate lymphoid cell type 1 (ILC1), is critical for intestina homeostasis and inflammation by regulating the numbers and functions of IEL and ILC1. We will use our newly generated CD160 knockout (CD160-/-) mice to test this hypothesis in the following Specific Aims. Aim 1: To investigate mechanisms by which CD160 contributes to intestinal homeostasis. I will mechanistically determine how CD160 regulates IEL numbers at steady state by analyzing IEL development, trafficking and retention within the intestinal epithelium. Since both IEL and ILC are thought to support epithelial cell homeostasis and barrier function at steady state, I will additionally test whether CD160-/- mice have a corresponding alterations in barrier permeability. Aim 2: To investigate the contribution of CD160 on IEL and ILC1 in controlling intestinal inflammation. Expression of CD160 in the intestine is highly specifi to IEL and ILC1, however we have yet to clarify the cell type-specific contributions of this co-inhibitory pathway. In this aim we will elucidate how CD160 signaling distinctly impacts IEL and ILC1 function and determine the relative impact of impaired CD160 signaling on IEL and ILC1 in two distinct models of intestinal inflammation. These studies will specifically address how CD160 expression on IEL and ILC1 can regulate intestinal homeostasis, inflammation and immunity. Additionally, these findings will provide important information about how modulating the CD160:HVEM pathway may therapeutically control intestinal disorders such as IBD and Celiac disease.